Demand compensated hydraulic system



June 30, 1959 J. R. ALLEN ETAL DEMAND COMPENSATED HYDRAULIC SYSTEM FiledJan. 2'7. 1958 Mkbkk m Wu QWQQS INVENTOR. J. R. ALLEN J.H. KRESS P. R.BUNNELLE United States Patent 2,892,312 DEMAND coMi ENsATE'DHYDRAUucSYSTEM *J'ames'R. Allen, Cedar Falls, and James H. Kress, Waterloo,Iowa, and Philip R. Bunnelle, San Jose, Calif 'assignors, by mesneassignments, to Deere 8: Com- .pany, a corporation of Delaware"Application January 27,-1958, Serial No. 711,360 10 Claims. (Cl. 60-52)This invention relates to a fluid pressure system and .more'particularly-to an improved system embodyinga demand-compensated principle in whichthe pump dis charge power from a variable-displacement pumpcan be madeproportional tothe-instantaneous load demand of one or more fluidmotorspowered by the pump.

In general, the important objects of theinvention are to "provide anarrangement-in which fluid power is pro- ,portional to the momentarydemand of the system in both pressure and volume, and particularly asystem hav- -ing acceptably high overall efficiency at all loadconditions, thereby-significantly reducing heat problems. This includesa standby phase in which power consumption is relatively low. It is afeature of the invention to apply the system to a plurality of motors,such as hyof motors.

The system ls-designed particularlyfor use in agricultural tractorswherein a number of hydrauliefunctions must be performed, such aspowersteering of the tractor front'wheelsandpower operated actuation of oneor more rockshafts for adjusting implements associated with the tractor.

In the drawings, a; variable displacement pump is shown at lll ashav'ing-mechanism 12 *by means of which the output of the pump may bevaried. The specific type of pump is not material, as longas it is any'ofthe well known variable displacement'types in'which the-stroking"mechanism 12 is arranged-so that its'output or stroke is increased by amechanical force, such asa compression spring 14 acting against a memberor piston-16 to move that member in the direction of the labeled arrow.The

' piston 16' is carried .in a cylinder '18 so' that its face is'subjectable to fluid pressureadmitted to the cylinder at the side ofthe piston opposite to thespring 14. The pump -10 is connected toreservoir in a suitable manner, an intake line bein'gsh'own' 'atI20' asbeing connected to a reservoir 22.

The-dischargeside of the pump 10 is'conne'cted via a pressure line 24ama motor control valve26 toa twoway fiuidpressuremotor 28. The motorcontrol valve "26 may bemannau and/erservo operated by any suitablemeans, represented here' schematically'at30. The valve likewise isillustrated schematically, as'are many of the well known hydrauliccomponents employed in the system. A typical flow control valve 32'is'in'te'rposed 26, for obvious reasons.

line .38.

ice

'2 in the pressure line 24 ahead of the motor control valve The motorcontrol valve .is shown in its neutral position and is of theclosed-center type, which permits no flow .when in neutral. The valve isconnected by apair otmotor lines 34 and 36 respec= tively to oppositeends of .the cylinder of the motor 28; When the motor control valve 26is shifted downwardly from its neutral position, the high ,pressure line24 is connected to the motor'line 34 so as to retract the motor 28 andreturn is via theline 36 to an exhaust or reservoir Movementofthe'control valve 26 to an upper position, just the opposite of thatjust described, will interchange the motor lines 34 and 36 so that the.latter is connected to .the pressure line 24 to extend the motor 28.In'the embodiment.from which the present system .is taken, the motor 28represents the hydraulic cylinder that is associatedwith the tractorsteering mechanism,.and is therefore subjected to little if any'residualload.

A basicpart of the demand-compensated system comprises a strokingmechanism control or demand valve means indicated in its entiretyby thenumeral 40 and comprising a cylinder 42 which forms a chamber carrying ashiftable valve piston 4-4 which separates the chamber into pressure andpilot sides 46 and 48 respectively. Biasing means in the form of arelatively light spring 5%) urges the piston 44 toward the pressurevside46 forthe purpose of controlling a port 52 which is permanentlyconnectedby a control? line 54to the cylinder 18 of the strokingmechanism, the connection of the line '54 being made to the cylinder 18at the side of the piston inoppositionto the spring '14. The line 54'hasa restricted connection or orifice56 to the reservoir 22. The ,pressureside 46 of the valve means 40 is connected by a line 24a to the pressureline 24 intermediate the discharge side of the pump 10 and the motorcontrol valve 26. To the extent thus far described, it will be seen thatthe valve piston 44 is subjected to twoforces; namely, pressure via theline 24 and .the spring load via the springSi). It will be further seenthat when the valve 44 is .open as respects the ports 52, pressure fromthe pressure line 24 is transmitted via 24a52-54 to thefluid-pressurereceivable end of the piston 16 in the strokingmechanism'12, so as to oppose the stroking mechanism-spring 14 for thepurpose of decreasing the. pump output.

In addition to the forces acting on the valve 44, .as just described,the-system .adds .pilotline pressure to the pilot side 48 .ofthedemandvalve meanschamber in conjunction with .the force'imposed .by thespring,150. This pilot line pressure isobtained through a pilot line .58which'hastwo branches 60 and-62 connected respectively to the ractorlines 34 and 36 between the motor control valve 26 andthe motor 28. Whenthe motor control valve 26 is in its neutral position, no flow orpressure occurs in the pilot line 58, but the reverse is true when themotor control valve is opened in either direction. In-order that pilotflow from one or the other of the branches 62 may not reverse itselfinthe other branch, depending upon which of the motor lines '34 and 36is pressurized, each branch carries apilot check valve,-respective1y at64 and 66.

From the description thus far, and ignoring the components notdescribed, it will be seen that when themotor control valve 26 is. inneutral, the pilot line pressureis zero. Therefore, the'pump dischargepressure via 24 and -24a.is balanced only against :the spring 50 at theopposite side of the valve piston 44, and these components arecalibrated to regulate IhCJSiIOkfi of'the pump ,10 via the mechanism12so'thata relatively small-standby pressure existsfin the line 24.

pilot pressure will occur in one of the other or the lines pump mustnecessarily be a fixed increment greater than the pilot pressure for abalance to exist. The magnitude of this increment is obviouslydetermined by the force of the spring 50.

The maximum pressure that the pump can produce is determined by pressurelimiting means indicated in it's entirety by the numeral 68 andcomprising a relief valve 70 biased to close against a pressure linebranch 72 by a spring 74. The value of the spring 74 is considerablyhigher than that of the spring 50. The pressure limiting means dumps viaa branch 76 to the control line 54 rather than directly to reservoir,and hence discharges into the mechanism 12 if an excess dischargepressure occurs, having the effect of decreasing the output of the pumpto zero.

After the motor control valve 26 is again closed or neutralized, thereis no fluid entering the pilot line, and the pilot pressure must reduceto zero in order to allow the system pressure to be reduced to itsstandby value. For this purpose, the pilot line 58 is connected toreservoir via a restriction or orifice 78. This prevents the trapping offluid in the pilot line between the bottom of the piston valve 44 andthe pilot line check valves 64 and 66.

The operation of the system to the extent described is as follows. Whenthe motor control valve 26 is open, fluid at standby pressure isadmitted to the motor 28 and simultaneously to the pilot line that isconnected to whichever motor line is pressurized. This pressure in thepilot line will signal for higher pump discharge pressure, and thispressure then increases the pilot line pressure which in turn signals anincrease of pump discharge pressure. During this build-up cycle, whichis very nearly instantaneous, practically no flow occurs. The increaseof pump discharge pressure will continue until the resistanceencountered by the motor 28 is overcome and the motor begins to move. Atthis point, appreciable flow begins so that there is a pressure dropacross the control valve 26 and in fact throughout the entire fiow path.Since the pilot pressure is taken in such a place where it is equal tothe pressure acting on the motor 28, it will signal the pressure actingon this motor only, and this signal will be transmitted to the pump viathe means 40. Since it is clear that no more pressure can act on themotor 28 than is required to move the resistance to which the motor isconnected, the pilot line pressure can reach no greater magnitude thanthat necessary to activate the motor. Since the pump discharge pressureis thus determined by the pilot pressure plus the fixed incrementestablished by the spring 50, the pump will discharge at a pre-setpressure increment above that required to move the motor. Theoretically,all this pressure increment should be dissipated across the inletmetering port of the motor control valve 26. As this valve is openedWider, allowing more flow to the motor, the stroking mechanism controlvalve means 40 will compensate by putting the pump into an increaseddisplacement, since the piston valve 44 will close the port 52 and fluidtrapped in the mechanism 12 below the piston 16 will escape to reservoirvia the orifice 56. This increased flow can continue until the motorcontrol valve 26 is opened to its maximum position, subject to the limitof the flow control valve 32. At this point, it should be noted that theflow control valve may be dispensed with, but is an expedientparticularly desirable in a multi-motor system.

As the motor control valve 26 is moved toward neutral,

4 the flow rate is decreased until it ultimately reaches-zero; When nofurther flow is admitted to the motor and thus to the pilot line 58, thepressure in the pilot line bleedsthrough the pilot line bleed orifice78, thereby reducing the pilot pressure to zero which in turn allows areduction: in the discharge pressure since the bottom of the piston!valve 44- is no longer subjected to any force in addition to the springforce 50. Since this spring force determines the standby' pressure, thesystem accordingly returns to standby operation;

As previously indicated, it is a feature of the invention to incorporatea plurality of motors in the demand-- compensated system. A secondmotor, here a one-way motor 80, is connected to a pressure lineextension 82 via a flow control valve 84 and a second motor controlvalve 86 under control of suitable manual and/or servo actuating meanssuggested at 88. The motor 30 is here representative of a hydrauliccylinder useful on an agricultural tractor for rocking a tractor-mountedrockshaft. The motor control valve 86 for the motor is connected to thatmotor by a motor line 90 and a pressure line 94 incorporating a checkvalve 92. A pilot branch 96 conmeets the pressure branch 94 to the pilotline 58 and includes a pilot line check valve 98 for preventing reversalof pilot flow when the pilot line 58 is pressurized by the motor 28.Similarly, the pilot line check valves 64 and 66 prevent reverse pilotflow when the pilot line is pressurized by operation of the motor 80.The motor control valve 86 is connected to reservoir at 100.

The system employs a third motor here, a two-way motor 102 controlled bya motor control valve 104 which may be connected to a suitable manualand/or servo actuator as at 106. This valve is connected to an extension108 of the pressure line 24-82 and a flow control valve 119 isinterposed in the line 108 ahead of the valve 104. One side of the valveis connected to reservoir at 112 and its opposite side is connected bymotor lines 114 and 116 respectively to opposite ends of the motor 102.Because the motor 102 is of the two-way type and is connected to arockshaft on a tractor, for example, it will be subjected to residualload and for this purpose it is desired to use pilot-operated checkvalves as at 118 and 120, respectively, in the motor lines 114 and 116.These check valves may be of any suitable type and the rectangles at 122and 124, respectively, indicate that these valves are hydraulicallyoperated. A pair of pilot line branches 126 and 128, respectivelyincorporating check valves 130 and 132, are connected respectively tothe motor lines 114 and 116, the system in this respect duplicating thearrangement previously described at 60, 62, 64 and 66.

When two or more motor control valves are opened simultaneously, thepressure will build up to the level required of the most heavily loadedmotor. The motor encountering the lighter load will move first and willaccelerate to the flow rate set by either its flow control valve (32, 84or or the amount of opening of its motor control valve (26, 86, or 104).There will be, at this time, no motion in the most heavily loaded motor,and therefore, the pilot signal will indicate the requirement for stillmore pressure until the most heavily loaded motor is moved. Pilotpressure in the line 58 will of course build up and oppose the pressureat 24-2411, the valve piston 44 moving up to cut 011 the port 52 andrelieving any counter pressure against the piston 16 in the strokingmechanism 12, whereby the pump discharge pressure will build up until itequals that required to move the most heavily loaded motor at a flowrate equal to the total of the number of motors in action. On the basisof the foregoing, any number of motors can be operated independently andsimultaneously to th limit of the pump capacity.

It will thus be seen that the demand-compensated hydraulic system is atype of closed-center arrangement that provide both pressure and flowrate propor- .pres'sure line causes a decrease in displacement.

tional to the instantaneous load demand condition. 'lhe fvariab ledisplacement pump "is automatically "controlled control valve is opened,the pressure in the signal "or pilot line will indicate the exactpressure (excluding line losses) needed to move the "motor. The'p'ilotpressure cannot signal a higher pressure than that needed 'toovercon1ethe resistance encountered by the'n'ioto'r. This resistance is'theresultant of two loadcornponents. First, there 'is the component due'tothe Lphysieal'load acting on the operating 'rrioto-r and is madeup'of'static load, inertiaand viscous resistance. Second there is acomponentfdue to the return line pressure. The'second component'isapplicable o'nly'infthe case of a'do'uble acting motor as at 28 and 102.The ump'wiu'preduce the nevi/rare hecessary to's'ustain the signaled p1""sure. Or, as compared to the pressuie-compensated arrangement whichwill produce the flow rate necessary "to maintain one iix'edpressure,the demand-compensated arrangement will produce any new from minimum "tomaximum at anypressure from standby to maximum.

As already indicated, the variable displacement pump may be of any type,and the stroking mechanism -12 is of a design such that an increase ofpressure in the 'As indicated, a spring or other mechanical device maycause the increase of stroke.

The stroking mechanism control or demand valve means 40 is actually adifierential typepressure transducer. The diflerential is pre-set by thespring 50 so that the pressure in the line 24a will bleed through theport 52 and control line until it becomes a set increment greater thanthe pilot pressure in the line 58. If the pressure at 24a or at thepressure side 46 of the valve 44 is greater than the pilot pressure (thepressure at the pilot side 48 of the valve 44, plus the spring force ofthe spring 50) the valve-44 moves downwardly, opening the port 52 to thecontrol line 54. If the values are reversed, the valve 44: moves up andcloses the port 52, allowing 'a reduction ofpressure in the line 54.When equilibrium has been reached, a small steady flow is metered to theline 54 via the port'52.

The magnitude of how allowed past'the transducer valve 40 determines thecontrol pressure inthe line 54, which cannot be greater than thatexisting in the line 24a, but can be any desired lesser value dependingupon the size of the fixed bleedorifice'SG.

On the basis of the foregoing, it will be'relatively easy tocalculateand design for certain desirable factors,

afiecting, for example, the magnitude of the controlflow to orifice 56.The size of this orifice is influenced by the fiow'necessary to maintainpump stability 'and the permissible time interval for the pump to alter"stroke. Other factors such as efiiciency, motor control valve designand type of actuationof the motor control valve can be varied as desiredwhile still enabling exploitation of the novel demand-compensatedsystem. These and other factors not categorically covered, as well asvariations in the' preferred sys'tem'disclosed, will' readily occur tothose versed in the art, all without departurefrom the: spirit and scope'of the invention.

What is claimed "is:

"1. A fluidlpressure'system er the class describedjcomprising: "avariable displacement pump having *output contmImec'hanism includinga"pis'ton, means biasing the piston in cne direction to increase :puinpoutput and a cylinder carrying the piston" for receiving fluid pressureto move the-piston inthe-opposite direction to decrease pump outputyafiuidpressure motor; a high pressure line connected between-thepump andmotor; a motor" control valve interposed in said line for selectivelyopening and "erasing s id line =to the'motor; deniand'vnve inicludin'g"-a chamber "and '"a piston valve shiftable therein se'p'a'fa rn said*charriber-into a pressure side coni'neet ed 'toithe "prs'sure li'lieahead of the motor control a pjilot sid'e connectedtos'aid pressure linebed fridtor "central valve and the motor, a part 'inthe chamber atsaidpressure side and openable by the piston valve 'in responseto-linepressure, and means biasing he'pistonvalve to 'cltises aid p'ortin opposition to line pressure and in conjiinetionwith pilot pressurewhen the morer'eontmlvalve is s'et to open the pressure line-tothe"motorfa restrieted-bleedline connecting the pilot 'side iSf thefdemarid*valve means to 'res'ervoir; a control line conne'eting said "to the pumcourier mechanism cylinder forsfipply g fiuid'from the'pr'es sure'sideof th'e'deniand Valveineans to the mechanism pistom'are's'tricted'res'er- 'voir' lirieleading from the control line toreservoir; and

pressure-limitin mean connecting the control line to'fthe p essure linebetween the ump and the demand valve -rneans "a'nd incIuding'a reliefvalve and biasingnieans closing said relief valve, said relief valvebiasing means being of greaterval'ue than-the biasing'meansin the-de'mand'valve means.

'ne'eted'to the pr'es'sure lineahead of the motor control valv'eaiid apilot-side connected to said pressure line-be- 't'ween's'aid'motorcontrol valve and the motor, a port in "the chamber at 's'aidpres'sur'eside and "openable by the piston valve in response to line pressure, andmeans biasing' the piston valveto close said port in opposition to 'linepressure andin conjunctionvvith pilot pressure when the motor controlvalve is set to open the pressure line *to the'mdtor; 'a restrictedbleed line connecting the pilot -"side-'o'f the demand-valve means toreservoir; a control 'line connecting said port to the pump controlmechanism 'for supplying fluid from the pressure side of the demandvalve means to the mechanism member; a restricted reservoir line leadingfrom the control line to reservoir; and pressure-limiting'meansconnecting the pressure line between the pump and the demand valve meansto reservoir and including a relief valve and biasing means closing saidrelief valve, said relief valve biasing means being of greater valuethan the biasing means in the demand valve means.

3 A fiuid pressure system of the class described, comprising: avariabledisplacement'pump having output control -rnechanis'm including a piston,means biasing the piston in one direction to increase pump output and "acylinder carrying the piston for receiving fluid pressure to move thepiston in the opposite direction to *decrease pump output; atwo-wa'yfluid pressure motor; a high pressure line connected to thepump; a motorcontrol valve connected to the high pressure line and having'a pair ofmotor lines connected respectively to opposite ends of the motor, saidmotor control valve being selectively settable to 'closethe pressureline or to pressurize either motor line'while connecting the other motorline to reservoir; demandwalve means including a chamber and apistonvalve shiftable'therein'and separating said chamberinto a'pressureside'connected to the pressure line ahead of the motor control valve anda pilot side'con- 'nected' to both motor lines for receiving pilot flowfrom 7 lines including a pair of check valves for preventing pilot flowfrom the pressurized motor line to the other motor line, said demandvalve means further including a port in the chamber at said pressureside and openable by the piston valve in response to line pressure aheadof the motor control valve and means biasing the piston valve to closesaid port in opposition to line pressure and in conjunction with pilotpressure when the motor control valve is set to pressurize either motorline; a restricted bleed line connecting the pilot side of the demandvalve means to reservoir; a control line connecting said port to thepump control mechanism cylinder for supplying fluid from the pressureside of the demand valve means to the mechanism piston; a restrictedreservoir line leading from the control line to reservoir; andpressure-limiting means connecting the control line to the pressure linebetween the pump and the demand valve means and including a relief valveand biasing means closing said relief valve, said relief valve biasingmeans being of greater value than the biasing means in the demand valvemeans.

4. A fluid pressure system of the class described, comprising: avariable displacement pump having output control mechanism including amember movable in one direction by fluid pressure to decrease pumpoutput and biased to move in the opposite direction to increase pumpoutput; a two way fluid pressure motor; a high pressure line connectedto the pump; a motor control valve connected to the high pressure lineand having a pair of motor lines connected respectively to opposite endsof the motor, said motor control valve being selectively settable toclose the pressure line or to pressurize either motor line whileconnecting the other motor line to reservoir; demand valve meansincluding a chamber and a piston valve shiftable therein and separatingsaid chamber into a pressure side connected to the pressure line aheadof the motor control valve and a pilot side connected to both motorlines for receiving pilot flow from whichever motor line is pressurizedby the motor control valve, said connection between said pilot side andmotor lines including a pair of check valves for preventing pilot flowfrom the pressurized motor line to the other motor line, said demandvalve means further including a port in the chamber at said pressureside and openable by the piston valve in response to line pressure aheadof the motor control valve and means biasing the piston valve to closesaid port in opposition to line pressure and in conjunction with pilotpressure when the motor control valve is set to pressurize either motorline; a restricted bleed line connecting the pilot side of the demandvalve means to reservoir; a control line connecting said port to thepump control mechanism for supplying fluid from the pressure side of thedemand valve means to the mechanism member; a restricted reservoir lineleading from the control line to reservoir; and pressure-limiting meansconnecting the pressure line between the pump and the demand valve meansto reservoir and including a relief valve and biasing means closing saidrelief valve, said relief valve biasing means being of greater valuethan the biasing means in the demand valve means.

5. A fluid pressure system of the class described, comprising: avariable displacement pump having output control mechanism including apiston, means biasing the piston in one direction to increase pumpoutput and a cylinder carrying the piston for receiving fluid pressureto move the piston in the opposite direction to decrease pump output; apair of fluid pressure motors; a 'high pressure line connected to thepump; a pair of motor control valves connected in parallel to thepressure line and respectively having independent motor lines leadingrespectively to the motors, each motor control valve being selectivelysettable to close or open the pressure line to its respective motorline; demand valve means including a chamber and a piston valveshiftable therein and separating said chamber into a pressure side con-.s V W 2,892,812

nected to the pressure line ahead of the motor control valve and a pilotside connected to both motor lines for receiving pilot flow fromwhichever motor line is opened tothe pressure line by its associatedmotor control valve, said connections to the motor lines including checkvalves operative respectively on the motor lines to prevent pilot flowfrom the opened motor line to the closed motor line, said demand valvemeans further including a port in the chamber at said pressure side andopenable by the piston valve in response to line pressure ahead of bothmotor control valves, and means biasing the piston valve to close saidport in opposition to line pressure and in conjunction with pilotpressure from whichever motor line is opened by its associated motorcontrol valve; a restricted bleed line connecting the pilot side of thedemand valve means to reservoir; a control line connecting said port tothe pump control mechanism cylinder for supplying fluid from thepressure side of the demand valve means to the mechanism piston; arestricted reservoir line leading from the control line to reservoir;and pressurelimiting means connecting the control line to the pressureline between the pump and the demand valve means and including a reliefvalve and biasing means closing said relief valve, said relief valvebiasing means being of greater value than the biasing means in thedemand valve means.

6. A fluid pressure system of the class described, comprising: avariable displacement pump having output control mechanism including amember movable in one direction by fluid pressure to decrease pumpoutput and biased to move in the opposite direction to increase pumpoutput; a pair of fluid pressure motors; a high pressure line connectedto the pump; a pair of motor control valves connected in parallel to thepressure line and respectively having independent motor lines leadingrespectively to the motors, each motor control valve being selectivelysettable to close or open the pressure line to its respective motorline; demand valve means including a chamber and a piston valveshiftable therein and separating said chamber into a pressure sideconnected to the pressure line ahead of the motor control valve and apilot side connected to both motor lines for receiving pilot flow fromwhichever motor line is opened to the pressure line by its associatedmotor control valve, said connection to the motor line including checkvalves operative respectively on the motor lines to prevent pilot flowfrom the opened motor line to the closed motor line, said demand valvemeans further including a port in the chamber at said pressure side andopenable by the piston valve in response to line pressure, ahead of bothmotor control valves and means biasing the piston valve to close saidport in opposition to line pressure and in conjunction with pilotpressure from whichever motor line is opened by its associated motorcontrol valve; a restricted bleed line connecting the pilot side of thedemand valve means to reservoir; a control line connecting said port tothe pump control mechanism for supplying fluid from the pressure side ofthe demand valve means to the mechanism member; a restricted reservoirline leading from the control line to reservoir; and pressure-limitingmeans connecting the pressure line between the pump and the demand valvemeans to reservoir and including a relief valve and biasing meansclosing said relief valve, said relief valve biasing means being ofgreater value than the biasing means in the demand valve means.

7. A fluid pressure system of the class described, comprising: avariable displacement pump having output control mechanism including apressure chamber operative to vary pump output according to changes inpressure in said chamber; a fluid pressure motor; a high pressure lineconnected between the pump and motor; a motor control valve interposedin said line for selectively opening and closing said line to the motor;demand valve means including a chamber and a piston valve shiftabletherein and separating said chamber into a pressure side connected tothe pressure line ahead of the motor control valve and a pilot sideconnected to said pressure line between said motor control valve and themotor, a port in the chamber at said pressure side and operable by thepiston valve in response to line pressure, and means biasing the pistonvalve to close said port in opposition to line pressure and inconjunction with pilot pressure when the motor control valve is set toopen the pressure line to the motor; a restricted bleed line connectingthe pilot side of the demand valve means to reservoir; a control lineconnecting said port to the pump control mechanism chamber for supplyingfluid from the pressure side of the demand valve means to said chamber;and a restricted reservoir line leading from the control line toreservoir 8. A fluid pressure system of the class described, comprising:a variable displacement pump having output control mechanism including apressure chamber operative to vary pump output according to changes inpressure in said chamber; a two-way fluid pressure motor; a highpressure line connected to the pump; a motor control valve connected tothe high pressure line and having a pair of motor lines connectedrespectively to opposite ends of the motor, said motor control valvebeing selectively settable to close the pressure line or to pressurizeeither motor line while connecting the other motor line to reservoir;demand valve means including a chamber and a piston valve shiftabletherein and separating said chamber into a pressure side connected tothe pressure line ahead of the motor control valve and a pilot sideconnected to both motor lines for receiving pilot flow from Whichevermotor line is pressurized by the motor control valve, said connectionbetween said pilot side and motor lines including a pair of check valvesfor preventing pilot flow from the pressurized motor line to the othermotor line, said demand valve means further including a port in thechamber at said pressure side and openable by the piston valve inresponse to line pressure ahead of the motor control valve and meansbiasing the piston valve to close said port in opposition to linepressure and in conjunction with pilot pressure when the motor controlvalve is set to pressurize either motor line; a restricted bleed lineconnecting the pilot side of the demand valve means to reservoir; acontrol line connecting said port to the pump control mechanism chamberfor supplying fluid from the pressure side of the demand valve means tosaid chamber; and a restricted reservoir line leading from the controlline to reservoir.

9. A fluid pressure system of the class described, comprising: avariable displacement pump having output control mechanism including apressure chamber operative to vary pump output according to changes inpressure in said chamber; a pair of fluid pressure motors; a highpressure line connected to the pump; a pair of motor control valvesconnected in parallel to the pressure line and respectively havingindependent motor lines leading respectively to the motors, each motorcontrol valve being selectively settable to close or open the pressureline to its respective motor line; demand valve means including achamber and a piston valve shiftable therein and separating said chamberinto a pressure side connected to the pressure line ahead of the motorcontrol valve and a pilot side connected to both motor lines forreceiving pilot flow from whichever motor line is opened to the pressureline by its associated motor control valve, said connections to themotor lines including check valves operative respectively on the motorlines to prevent pilot flow from the opened motor line to the closedmotor line, said demand valve means further including a part in thechamber at said pressure side and openable by the piston valve inresponse to line pressure ahead of both motor control valves, and meansbiasing the piston valve to close said port in opposition to linepressure and in conjunction with pilot pressure from whichever motorline is opened by its associated motor control valve; a restricted bleedline connecting the pilot side of the demand valve means to reservoir; acontrol line connecting said port to the pump control mechanism chamberfor supplying fluid from the pressure side of the demand valve means tosaid chamber; and a restricted reservoir line leading from the controlline to reservoir.

10. A fluid pressure system of the class described, comprising: avariable displacement pump having output control mechanism including apressure chamber operative to vary pump output according to changes inpressure in said chamber; a fluid pressure motor; a high pressure lineconnected between the pump and motor; a motor control valve interposedin said line for selectively opening and closing said line to the motor;a control line connected to the high pressure line ahead of the controlvalve and leading to the control mechanism chamber; a pilot lineconnected to the high pressure line between the control valve and motor;a diflerential pressure transducer for regulating the pressure and flowin the control line to vary the pressure in the control mechanismchamber, said transducer having pre-set therein a predetermineddiiferential enabling the transmission of pump pressure to said chamberat a predetermined value when the control valve is closed whereby thepump output is a predetermined value, said transducer being connected tothe pilot line and responsive to pressure therein when the control valveis opened for changing pressure in the control line so as to vary pumpoutput in accordance with demand on the motor; and a restricted bleedline leading from the pilot line to enable the escape of fluid therefromwhen the control valve is closed so that the transducer will again incurthe aforesaid predetermined value of pressure in said chamber.

References Cited in the file of this patent UNITED STATES PATENTS2,408,303 Ernst Sept. 24, 1946 2,432,305 Geiger Dec. 9, 1947 2,716,946Hardy Sept. 6, 1955

